1. Field of the Invention
The present invention relates to a polyolefin-based composite material which is excellent in impact resistance and heat resistance, and which is applicable to automobile parts, household electrical product materials, aircraft parts, and construction materials. The present invention also relates to a process for producing such a polyolefin-based composite material in a very simple and economical manner in comparison with conventional processes.
2. Description of Related Art
In order to improve properties of polypropylene and other polyolefin resins, mechanical properties and heat resistance in particular, mixing and melt-kneading of a highly rigid inorganic filler has been hitherto considered. Since a polyolefin resin composition with an addition of an inorganic filler exhibits excellent rigidity and heat resistance, such a polyolefin resin composition has been widely employed as a molding material, particularly in the fields of automobile parts and household electrical products.
It has been known that performances of a composite material, such as rigidity, impact resistance, and heat resistance, can be enhanced by using a filler finely prepared to the submicron level, and controlling the dispersion of the filler (see Japanese Patent Application, First Publication (Kokai), No. Hei 7-157592).
According to this process, however, since repeated grinding steps and classification steps are required in order to obtain the extremely fine filler grains, the filler is expensive, and this process is thus not economical.
Another process has been known in which a filler is dispersed into a resin composition by introducing an amine into a compound having a stratified structure so as to widen gaps between strata, thereafter inserting a desirable monomer in the gaps, and polymerizing the monomer; the clay mineral having a stratified structure is thereby allowed to collapse by itself using the energy generated during the polymerization, and dispersing the clay mineral at the nanometer level (for example, Japanese Patent Application, First Publication (Kokai), No. Sho 62-74957; Japanese Patent Application, First Publication (Kokai), No. Sho 64-9202; etc.).
However, although this process permits uniform dispersing of filler with a great efficiency, the polymerization reaction is limited to radical polymerization or condensation polymerization. Furthermore, since the monomer introduced into the gaps between strata, which is an olefin monomer, such as ethylene, propylene, butene, and isoprene, is non-polar, the monomer must be liquefied, and the polymerization reaction must be conducted under high pressure or in a highly concentrated condition, so as to leave the monomer consistently in the gaps between strata. However, even in a homopolymerization or copolymerization system, it is difficult to use the above monomers in reality. Moreover, since a special polymerization facility is required for the polymerization of the monomer, the high fixed cost included in the product price makes this process uneconomical.
Furthermore, application of the above process to an anionic coordination polymerization system or radical polymerization system, which is taken for polyolefin, would be inappropriate, since active hydrogen in an amino acid or the like which exists in the gaps between the strata acts as a catalytic poison in the reaction atmosphere, making it difficult to increase the molecular weight and to control the tacticity with regard to the polymer yielded.
Yet another process has been known as a means for dispersing a filler into polyolefin, according to which a silicate compound in which an onium salt such as an ammonium salt is retained is employed (see Japanese Patent Application, First Publication (Kokai), No. Hei 6-41346, and Japanese Patent Application, First Publication (Kokai), No. Hei 7-70357).
According to such a process, a compound which is swelled and cleaved to form the stratal gaps of 5 nm or larger by means of a swelling/dispersing medium made of an organic solvent and the like should be used in order to finely and uniformly disperse the stratiform silicate which is made lipophilic by an onium salt. However, removal of the solvent would make this process impractical from an environmental perspective, and kneading in the presence of the solvent would degrade the productivity, and would make this process uneconomical. Furthermore, complete removal of the solvent or the like in the obtained composite material is difficult, and adversely affects the performances of the composition, such as heat resistance and rigidity. Therefore, it has been desired that filler be dispersed finely and uniformly into polyolefin without using a solvent.
That is to say, although it has been possible to finely and uniformly disperse a filler into polyamide, dispersion of such a filler in polyolefin has not hitherto be practical.